This invention relates to a spacer of an optical fiber cable for supporting and protecting a plurality of optical fibers and a method for forming such a spacer.
An optical fiber cable for telecommunication purposes is well known in the art and typically comprises a plurality of optical fibers arranged around a tension member.
In one of the known optical cables, the tension member is sheathed by a spacer body which is formed of thermoplastic resin and is provided with a plurality of longitudinally extending grooves for retaining optical fibers therein. Accordingly, strict dimensional accuracy of the grooves is required in order not to affect transmission characteristics of the optical fibers. Such spacer is made by extruding molten thermoplastic resin around the tension member through a stationary or rotary die having an opening of desired shape and then solidifying it by cooling. The grooves are formed in accordance with the shape of the die opening and, in the case of use of a rotary die, helical grooves are formed longitudinally on the spacer. The diameters of the tension member and the spacer body, depth and number of the grooves, etc. depend on the requirements of the optical fiber cable to be manufactured, and spacers of various designs and dimensions have been used.
However, the above method has the following problems. That is, when a ratio of the diameter of the tension member, which diameter is determined by a required tensile strength, to the root diameter (dimension between two opposed groove bottoms) of the spacer body is relatively small and when the spacer body is composed of a single coat of thermoplastic resin around the tension member, the grooves or spacer body often get deformed as shown in FIG. 3A and it is difficult to attain the desired dimensional accuracy with good yield. This tendency is very remarkable when a crystalline thermoplastic resin such as polyethylene or polypropylene is used. This is believed to be caused by a drastic shrinkage due to crystallization when the crystalline thermoplastic resin is cooled and solidified.
The inner part of the spacer body takes more time to solidify than the peripheral part thereof so that the inner part, during its solidifying process, pulls the peripheral part which has already been solidified to a great extent. Further, in the method of forming the helical grooves by rotating the die, it is much more difficult to attain the dimensional uniformity of the grooves because the stress is applied to the extruded resin differently than when forming linear grooves.
Various materials such as metal wires, high strength synthetic fibers and fiber reinforced plastics (FRP) have been proposed to be used as the tension member, and glass fiber reinforced plastic is particularly suitable for this purpose because of its small specific gravity, large tensile strength, non-electroconductivity and coefficient of thermal expansion and flexibility similar to those of the optical fibers. An adhesive strength between the tension member and the spacer body must be sufficiently large so that they will not separate from each other due to change of atmospheric temperature to which the cable is subjected. Based on a coefficient of thermal expansion of usually employed thermoplastic resins, and assuming that the cable is subjected to a temperature change of 40.degree. C., it is necessary that the above-mentioned adhesive strength be more than 60 kg/cm.sup.2 in order to avoid the separation which would destroy the function of tension member. However, the known spacer in which the tension member of glass fiber reinforced plastic is first hardened by passing it through a molding die and is then sheathed by the spacer body, has only an insufficient adhesive strength of about 30 kg/cm.sup.2 at the most. This is because of a smoothness of the inner surface of the molding die and a consequent smooth outer surface of the tension member, the smooth inner surface of the molding die being required in order to reduce resistance during a drawing operation. Consequently, in the known spacer a main factor that prevents the spacer body from separating from the tension member is the shrinkage force of the spacer body created when it is solidified.
Such a poor adhesive strength involves a possibility that the helical grooves of the spacer body are displaced relative to the tension member during arrangement of the optical fibers, resulting in various troubles due to incorrect phase. Further, the spacer body of thermoplastic material, which has a coefficient of thermal expansion larger than that of the tension member, would act on the optical fibers to impart thermal stress thereto as the atmospheric temperature changes, causing an increase of transmission loss due to microbending.